JOSM has many controls and data validations on "commit". I'm notified when something seems to be wrong and I can fix it immediately. iD has nothing like this.

Using of any external tool that is updated next day, is not much comfortable. You need to not forget and have a time to do it.

There are some possibilities:
1) implement basic validations to iD
2) implement basic validations to OSM API
3) implement a special layer on main osm.org page, so any error became visible almost immediate.

You are exactly right of course. I use JOSM exclusively with its useful error detection capability.
I have not checked it but I suspect that most errors are generated using Potlach and ID.
JOSM is so easy to use. It makes me wonder why a lot of contributors don't graduate to it.

JOSM can only check what it has. If I don't download everything in an area I'm working on I can break relations and not get any warning from JOSMs validator.

OSMinspector is one of the QA tools .. there are others too. I do use OSMinspector (keeping track of my own errors!) .. and I fix some errors there (mine + other peoples).. sometimes I make changeset comments to try and inform others of the problem so they might be more aware. Some times contributors are not aware of these QA tools, beginners don't get to 'see' them. Osmose tends to have things that I cannot fix without a lot of on ground work. So many of these have to wait.

Actually, that editor that you don't know how to spell has the QA cues built into the UI, not as an afterthought at upload time - see for example http://www.openstreetmap.org/#map=18/56.51881/-5.75593 , where the duplicate nodes appear as red pustules saying "fix me now!".

More seriously, the reasons why contributors don't correct their own work include:

They're new editors, still finding their way. Everyone needs to be allowed to get it wrong a few times as they learn how to edit OSM (whatever editor they are using).

No-one has actually told them that something is a problem. If people don't know about QA tools they won't know that they exist. The best way to get in contact with new mappers is to comment (politely!) in a changeset discussion. Anything that just sounds like "you're doing it wrong!" is unhelpful, even if they really are doing it wrong.

The QA tool is wrong. JOSM's validator is good, but still "cries wolf" quite a lot of the time, because not everywhere in the world is like Germany.

What's been uploaded is only the "first draft". For example, last night I uploaded this. It was important to upload it because it's a major change to the area, but it's far from complete, and there will be many things in there that need refining based on further survey. JOSM's validator there finds 1 error and 186 warnings, most of which are indications that further survey is needed (e.g. highway crossing building is obviously wrong, but which is in the right place?). Some of what JOSM's validator says there are just completely invalid, and it's up to the mapper to work out which and why.

Sorry. Initially I've spoken about iD only. This is the editor recommended to beginners and there are no validation checks. So new user does not know, that something is wrong. Editors should be more pedantic and help users with common data errors as much as possible.

The best way to get in contact with new mappers is to comment (politely!) in a changeset discussion.

More seriously, a comment such as "you have broken a multipolygon here" will not communicate anything to someone who does not know what a multipolygon is.

Of course, there are genuine issues with e.g. MAPS.ME users not replying to comments. Part of that is because MAPS.ME users sometimes don't even know what OSM is, so to find it "talking to them" is very unexpected. Also matching the language of the comment to the language of the mapper is important (not everyone speaks English, and web translation these days is good enough that they'll at least get the sense of the problem, so there's no excuse not to).

All the above comments are correct (even the one about spelling, but that does not bother me at all). Using OSMI and Bing I do a lot of correcting and when I see a consistent error being made I do contact the pertinent contributor (with a link to OSMI): Only about 5% reply and in these cases most are thankful for the information.

iD has validation checks, they're just relatively simple (like unmarked ways, and points without any tags). I think its validation features should be enhanced, but probably not too much (we really don't want people to start faking data just to pass a validator).

In theory, it would be nice if everyone could JOSM, but it really sucks with high-DPI screens, and it's only user-friendly if you already have a fairly solid understanding of how OSM works, which is a total no-go for most people who are not armchair mappers.